In metal container manufacture, after the can body is completely configurated and decorated, but before the end is assembled onto the body, the interior surface of the can body is coated with a protective coating of a synthetic resin material.
It has been the conventional practice in the metal container industry to apply, as the coating material, a heat hardenable resin mixed in a volatile organic solvent which is sprayed into the open end of the container body and onto the interior walls thereof. The wet coated container is then passed through an oven in which hot air is circulated to evaporate the solvent and harden the coating.
Although organic solvent based resin coatings are widely used in the manufacture of metal food and beverage containers, one drawback to the use of these coating systems is that in the step of heat hardening the resin coating material, the solvent with which it is mixed is driven off and exhausted into the atmosphere, thereby giving rise to an atmospheric pollution problem. Within the last few years there has been a great deal of attention to atmospheric pollution and the atmospheric contamination which results from industrial exhaust. One of the contaminants at which attention is now directed is the solvent exhausted from the curing ovens of metal container manufacturers.
One method of avoiding the air pollution problem associated with organic solvent based coatings that has been considered by container manufacturers is to substitute an aqueous dispersion medium for the organic solvent presently used. Water dilutable acrylic resins, especially epoxy cured acrylic resins, have been considered as aqueous dispersible materials for coating metal containers but have the drawback that the coating material readily absorbs various odors from its surroundings. After packaging liquid comestibles such as beer in containers internally coated with epoxy cured acrylic resin based coatings, the odors are released into the beer, and the delicate flavor of the beer is deleteriously affected.